The piston-type internal combustion engines of motor vehicles, in particular passenger vehicles, which are provided with a turbocharger controlled via a blow-off valve exhibit for specific load conditions a more or less large xe2x80x9cturbo hole,xe2x80x9d meaning a lack of rotational moment. This turbo hole is steady in the low-speed range and unsteady following a jump in the load. In order to keep the unsteady turbo hole small, the boost pressure control must be designed in such a way that following a load demand by the driver, the fastest possible boost-pressure buildup occurs. As a result of the system feedback, a highly progressive boost-pressure increase occurs. An increase in the boost pressure also causes an increase in the exhaust gas energy available for driving the charging turbine. If the acceleration occurs from low gears and starting from low speeds, this can result in a sudden strong increase of the boost pressure and thus to a sudden increase in the rotational moment, even if the driver keeps the pedal position constant at that moment. This is not only considered uncomfortable, but with extremely high performance vehicles can result in danger, owing to a sudden, strong acceleration or spinning of the wheels and a swerving of the vehicle. These problems cannot be solved through specifying a limit for the boost pressure in the air intake tract and a subsequent guided actuation of the blow-off valve. The danger and the impression of discomfort is not caused by the high engine moment per se, but by the increase in the engine moment that is not expected by the driver as a result of the delay.
It is therefore the object of the invention to specify a method for controlling the boost pressure, which makes the buildup of boost pressure more comfortable, but which also reacts quickly to sudden load demands.
This object is solved according to the invention with a method for controlling the boost pressure of a piston-type internal combustion engine with turbocharger. With this method, a blow-off valve installed in the exhaust gas tract upstream of a charging turbine is controlled by an engine control, which also detects the pedal position and a load-proportional gradient in the event that an acceleration action is initiated. If a predetermined value for the load-proportional gradient is exceeded, the blow-off valve is actuated in an opening direction such that a predetermined controlled pressure increase occurs.
Any measured value of the piston-type internal combustion engine, which is detected in dependence on the time and permits a statement on the change in the respective load condition of the engine in the broadest possible sense, is considered a xe2x80x9cload-proportional gradientxe2x80x9d within the meaning of the invention. These measured values primarily include the time-dependent change in the boost pressure in the air intake tract, the time-dependent change in the air mass flow or a directly or indirectly determined engine moment. It is also possible to predetermine a model value that is formed from measuring and control variables in the engine control for a time-dependent detection of the engine load.
In addition to specifying an absolute value of the measured value that is intended with the existing control, the measure according to the invention consists in detecting the gradient for this load-proportional measured value, meaning its change per time unit, and in specifying a limit value for this load-proportional gradient. In the even that the limit value is exceeded, the control returns the detected load-proportional gradient to the predetermined limit value by adjusting the blow-off valve in the xe2x80x9copenxe2x80x9d direction and, depending on the design of the complete system, by also reducing the load control of the engine. The load control intervention can be effected, for example, by adjusting the throttle valve, but also with unconventional methods such as variable valve control times or a mixture control.
As a result, the driver can adequately react to the strong increase in the boost pressure, which results in an increase in the rotational moment of the engine. It is thus avoided that the driver is surprised by an excessively rapid increase in the boost pressure. The rapid increase causes an undesirably high pressure in the air intake tract during the driver reaction time and thus a power output that is not desired or expected by the driver and results, for example, in a spinning of the drive wheels or a forward jump of the vehicle. The driver is given the opportunity to react to the increase in the engine moment with a reduction in the pedal load or even through dynamic driving actions (counter steering).
The operating conditions of the piston-type internal combustion engine and the turbocharger of a motor vehicle change according to specific rules because of the unsteady operation as well as the existence of the gearshift. Thus, it is provided according to one advantageous embodiment of the invention that the respectively predetermined limit values for the load-proportional gradient are taken from a performance characteristic of the engine control. For example, if the pedal is fully depressed at low speeds and in a low gear for an acceleration operation, other values result for the turbocharger behavior and thus also for the rotational moment change of the piston-type internal combustion engine than if this action occurs at a higher speed with a different gear. The same is true for the effect of these changes on the comfort perception of the driver. These differences are taken into account by specifying corresponding performance characteristics in the engine control.
According to one embodiment of the invention, the pedal gradient is furthermore taken into account as a limit value for the load-proportional gradient. The term xe2x80x9cpedal gradientxe2x80x9d within the meaning of the invention relates to the change in the pedal position in dependence on the time. Thus, if the driver pushes down the pedal only slowly to increase the vehicle speed, or keeps it at a constant position, the driver expects only a slow change in the engine moment and the limit value selected for the load-proportional gradient must be small. If the pedal is pushed down rapidly, the driver expects a correspondingly rapid change in the engine moment. A large load-proportional gradient can be permitted without loss of comfort or safety.
According to another advantageous embodiment of the invention, the respective limit value is taken from the performance characteristic for the pedal gradient that is low-pass filtered in a negative direction. This link favorably takes into account the expectations of the driver. With a constant pedal value, meaning the pedal gradient is zero, the driver normally expects only a moderate change in the rotational moment, even if the pedal value is high. In contrast, if the pedal is pushed down quickly, the driver expects a rapid rotational moment increase that corresponds to the speed of the pedal movement. However, this increase is less and less the farther back the pedal movement for acceleration occurred. As a result of the low-pass filter for the signal xe2x80x9cpedal gradient,xe2x80x9d the pressure increase in the air intake tract that is permitted by the engine control with increasing passage of time between the signal and the response of the turbocharger is lower than would actually be possible. As a result, it is avoided that the driver is surprised by an unexpected, sudden increase in the rotational moment. This value can be modified further in dependence on additional input variables.
According to another advantageous embodiment of the invention, the boost pressure gradient in the air intake tract is detected as the load-proportional gradient. The air mass flow in the air intake tract is proportional to the pressure in the air intake tract and also proportional to the load. As a result, the boost pressure gradient represents a load-proportional gradient that is directly connected to the respective control and regulation actions. If the driver demands a strong acceleration via the pedal position, then the increased fuel supply causes the piston-type internal combustion engine to react by increasing the exhaust-gas energy, which leads to an increase in the boost pressure via the charging turbine and the charger. By detecting the boost pressure gradients, the time curve for the boost pressure increase can be detected and, in case of an undesirably steep increase, the blow-off valve can be adjusted practically extrapolating in the opening direction, so that the increase will become less steep. As a result of the control behavior of the complete system involving the piston-type internal combustion engine and the turbocharger, a corresponding performance characteristic can be provided for the respective load condition and operating condition in the engine control. Thus, if the limit value for the boost pressure gradient that is respectively specified by the performance characteristic is reached, the blow-off valve can be correspondingly opened and the energy supply to the charge turbine can be reduced. It is thus possible to have a sensitive guidance of the boost pressure increase in the air intake tract. In driving situations where the pedal movement leads to the assumption that the driver desires and expects this, the physically possible unsteady performance of the system can be demanded. In situations where it can be assumed that the driver does not expect considerable changes in the rotational moment, the system dynamic can be suppressed to a comfortable and secure measure.
The method according to the invention and the above-discussed embodiments in principle can be used for piston-type internal combustion engines with a choke-free load control as well as a load control effected by a throttle valve in the air intake tract. According to another embodiment of the inventive method, the respective throttle valve position is detected with a control effective via a throttle valve and, if the predetermined value for the boost pressure gradient is exceeded, the throttle valve is adjusted at least at times in the closing direction. A boost pressure increase, which cannot be prevented by adjusting the blow-off valve due to the long idle times of the xe2x80x9cblow-off valve/turbochargerxe2x80x9d system, can thus be caught with this control action in the air intake tract. An undesirably steep increase in the engine load can therefore still be avoided and the response time of the system can be shortened noticeably. An undesirably rapid increase in the boost pressure is xe2x80x9ccaughtxe2x80x9d in the air intake tract already and a sudden rapid increase in the boost pressure is avoided even with a desired strong acceleration. For a piston-type internal combustion engine with a choke-free load control, e.g., via variably controlled cylinder valves, the undesirable boost pressure increase is correspondingly xe2x80x9ccaughtxe2x80x9d by changing the valve control times via the engine control.